Hybridoma cell lines have been recovered from spleens of adult F1 mice inoculated with parental spleen cells. These lines produce antibodies which react with viral envelope determinants of a wide variety of endogenous MuLV. Monoclonal antibodies from (B6 X D2)F1 mice inoculated with D2 spleen cells react predominantly with endogenous xenotropic MuLV. Antibodies recovered from (B6 X D2)F1 recipients of B6 cells and (NFS X AKR)F1 recipients of AKR cells react with determinants expressed by MCF viruses but not with any of the xenotropic, endogenous ecotropic or exogenous ecotropic MuLV in our panel. Several of these antibodies react with a gp70 molecule expressed on the membrane of a small population of cells (1-2%) in the spleen and bone marrow of adult mice and up to 70% of the cells from fetal liver. These antibodies appear to define a group of determinants unique to the erythroid lineage. The viral encapsidation of the relevant sequences appears to be under rather stringent control, as to date no infectious virus has been recovered from mouse tissues which expresses the entire spectrum of "erythroid specific determinants".